I tried to measure a 7915 regulator on my HP3478A 5.5 DMM. With correct polarity (Lo=-15V & Hi=GND), the reading is -15.82xx volt, but if I reverse the test lead (Lo=GND & Hi=-15V) the reading becomes +15.63xx volt! According to service manual, there is no calibration on negative DC, and all calibrations on DC is only based on positive reference. So, I don't think a full calibration will solve this problem. Is there anyone having this problem?
One of the tests for a correctly working voltmeter is to check it reads exactly -Value = +Value when reversed.
It sounds like your meter has developed an offset. What do you get when you short the inputs to the meter?
I don't know if that's a bench type or a hand held. If the former, it could be leakage to the power line. Does it have a ground lead or are both leads floating?
It's a bench type DMM. When both leads are shorted, it is about -0.0014mv ~ -0.0019mv. I will try a 12V car battery next time.
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Any battery will do... 1.5 or 9 volt etc.
It's possible the error measuring on the reg is down to some noise etc on the rails or AC "noise" on the chassis (ground), so reading a battery will prove beyond doubt. I would guess with a meter like yours it should be equal to within the last digit or so.
It's possible the error measuring on the reg is down to some noise etc on the rails or AC "noise" on the chassis (ground), so reading a battery will prove beyond doubt. I would guess with a meter like yours it should be equal to within the last digit or so.
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